`fish.length` <-
function(data,ncol=c(1,2,3,4))
	{
        if (is.null(data)) stop("argument 'data' must be a matrix with at least 4 columns (year, zone, sex, lenght)")
        if (!is.null(data)) 
		{
		if (dim(data)[2] < 4) 
			{
			stop("Argument 'Data' must be a matrix with at least 3 columns (year, zone, sex, lenght)")
			}
		else  {
			length <- cbind(data[ncol[1]],data[ncol[2]],data[ncol[3]],data[ncol[4]])
			names(length) <- c("year","zone","sex","length")
			print(paste("Your colection data is (Year x Zone):"))
			print(table(length [,1],length [,2]))
			}
    		}
	length.data <- list(length=length,frec.Table=table(length [,1],length [,2]))
	return(length.data)	 
	}

